1. Field of the Invention
The present invention relates to an image information reading apparatus having a reading unit for applying stimulating light in a main direction to a stimulable phosphor sheet which stores the radiation image information of a subject recorded therein and reading, with a line sensor, light that is emitted from the stimulable phosphor sheet in response to the stimulating light applied thereto, and a scanning system for feeding the stimulable phosphor sheet relatively in an auxiliary direction which is substantially perpendicular to the main direction.
2. Description of the Related Art
There is known a system for reproducing the radiation image information of a subject such as a human body or the like on a photographic photosensitive medium or the like or displaying the radiation image information on a display unit such as a CRT (Cathode Ray Tube) or the like, using a stimulable phosphor which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of the radiation, and, when subsequently exposed to applied stimulating light such as visible light, emits light in proportion to the intensity of the stored energy of the radiation.
Specifically, in the above known system, the radiation image information of a subject such as a human body or the like is temporarily stored in a stimulable phosphor sheet which comprises a stimulable phosphor layer in the form of a sheet, and then stimulating light such as a laser beam is applied to the stimulable phosphor sheet to cause the stimulable phosphor sheet to emit light representing the stored radiation image information. The emitted light is photoelectrically read to produce an image signal. Based on the image signal, the radiation image information of the subject is outputted (reproduced) as a visible image on a recording medium such as a photographic photosensitive medium or the like or a display unit such as a CRT or the like.
The system incorporates a reading unit having a stimulating system for applying stimulating light to the stimulable phosphor sheet and a light collecting system for photoelectrically reading light emitted from the stimulable phosphor sheet in order to scan the stimulable phosphor sheet two-dimensionally. The reading unit has a reading/erasing unit reciprocally movable in an auxiliary direction (auxiliary scanning direction) in confronting relation to the stimulable phosphor sheet, as disclosed in Japanese laid-open patent publication No. 2001-133919. The reading/erasing unit includes the stimulating system, a line sensor (the light collecting system) which comprises an array of photoelectric transducers such as CCDs (Charge-Coupled Devices) arranged in a main direction (main scanning direction), and an erasing device.
With the above arrangement, in order to accurately read the radiation image information carried on the stimulable phosphor sheet, it is necessary to accurately set the distance between the stimulating system and the stimulable phosphor sheet, and the distance between the line sensor and the stimulable phosphor sheet.
Specifically, if the distance between the stimulating system and the stimulable phosphor sheet varies while the radiation image information is read from the stimulable phosphor sheet, the stimulating light is brought out of focus on the stimulable phosphor sheet. The depth of field by the line sensor needs to be kept in a range of ±100 μm. If the distance between the line sensor and the stimulable phosphor sheet varies, then the depth of field varies, resulting in a failure to obtain the radiation image information of the subject with high accuracy. Even if the above distances do not vary, the overall quality of the reproduced image is lowered when the distances are too small or too large.
Therefore, it is necessary to individually increase the accuracy of the reading/erasing unit, the planarity accuracy of the reading/erasing unit in the auxiliary direction, and the planarity accuracy of the stimulable phosphor sheet. However, increasing the above accuracies tends to result in a substantial increase in the cost of the system, and the accuracies may possibly fail to be kept at a desired high level.